This invention relates generally to vortex-type flowmeters, and more particularly to a sensing system for such meters having a high degree of noise immunity.
A vortex-type flowmeter is adapted to measure the flow rate of a fluid passing through a conduit by generating fluidic pulses or oscillations whose repetition rate or frequency varies in accordance with flow rate. Two species of vortex-type meters are commercially available in the United States, one being the so-called Swirlmeter type and the other the bluff-body type. The present invention is not limited to these specific types and is applicable to all forms of hydrodynamic oscillatory metering devices in which fluidic variations are sensed to provide an indication.
In Swirlmeters, such as that described in U.S. Pat. No. Re. 26,410, a homogeneous fluid whose flow rate is to be measured, is forced to assume a swirl component. This is accomplished by feeding the fluid into the inlet section of a flow tube having a fixed set of swirl blades therein which imparts a swirling motion to the fluid passing therethrough. Downstream of the swirl blades in the tube is a Venturi section which first constricts and then expands the flow passage to transform the swirling motion into precessional movement in the expanding region of the Venturi section to create a vortex.
In commercially available Swirlmeters, detection of the cyclic variations is effected by means of a sensor probe mounted in the body of the meter transversely with respect to the longitudinal axis of the meter in the area where the vortex precession is near the inner wall of the flow tube. At the tip of the sensor probe is a heated thermistor which serves to detect the frequency of precession.
In the Swirlmeter, the thermistor is caused to operate in its self-heat region by applying a constant current thereto to heat the thermistor to a temperature above that of the fluid flowing through the meter. For a given fluid velocity, the thermistor is caused by the fluid passing thereby to undergo an appreciable increase in resistance by means of the cooling effect produced by the fluid stream.
Inasmuch as the current applied to the thermistor is maintained constant, an increase in its resistance will be effective as an increase in voltage. Any increase in velocity such as that produced by a fluidic vortex will further cool the thermistor, giving rise to a further increase in voltage. The voltage variations developed in the thermistor circuit as a result of the cyclic variations in local fluid velocity, have a frequency depending on flow rate and constitute the output signal.
In the bluff-body type of vortex meter, such as that described in U.S. Pat. Nos. 3,116,639 and 3,587,312, the body is mounted within the flow conduit transversely with respect to the flow axis thereof to create fluidic oscillations whose frequency is proportional to flow rate. In U.S. Pat. No. 3,587,312, these fluidic oscillations are detected by a pair of heated thermistors operating in a manner similar to that disclosed above in connection with the Swirlmeter.
Existing thermistor sensors for vortex-type meters have several disadvantages, among which are fragility, poor frequency response and relatively large power requirements, as well as high cost and limited commercial availability.
In the prior Herzl patent No. 4,015,472, there is disclosed a vortex-type flowmeter in which a piezo-electric sensor is actuated by vortex-pressure pulses to produce an alternating voltage at a frequency corresponding to the pulse frequency. This alternating voltage is processed to produce a corresponding square-wave signal which acts, by way of an electronic switch, to control the charge/discharge action of a set of capacitors. This circuit is so arranged that the average D-C switch current is directly proportional to the frequency of the square-wave signal and this, in turn, is directly proportional to the frequency of the sensed vortices.
The average D-C switch current is filtered and then applied to a summing terminal at the input of an operational amplifier whose output is connected through a feedback resistor to a two-wire transmission line leading to a remote receiving station containing a D-C power supply and a current-sensing element connected in series with each other and with the transmission line to produce a varying output current in a useful range (4 to 20 mAdC) which is an index to the flow rate.
A similar piezo-electric sensor arrangement is disclosed in the Richardson, et al U.S. Pat. No. 3,948,098 in which the piezo-electric signal controls the charge/discharge characteristics of a set of capacitors to produce an average charging current that reflects the frequency of vortex shedding and controls an output amplifier in a feedback arrangement to produce a D-C flow signal over a two-wire transmission line leading to a remote station.
In the Richardson, et al patent whose disclosure is incorporated herein by reference, the opposing electrodes of the piezo-electric sensing element are connected through respective input resistors to the input terminals of an operational amplifier. Since piezo-electric sensors suitable for this arrangement are limited by practical considerations to very small sizes, the capacitance of such sensors is quite low - in the general order of a few picofarads (pf). This dictates a very high input impedance for the associated operational amplifier and the input resistors must therefore be of very large value (in the megohm range).
The input impedance of the operational amplifier must be equal to the capacitive reactance X.sub.c of the piezo-electric element at its minimum operating frequency. Hence, if the piezo-electric sensor has a capacitance of 7500 pF and its minimum operating frequency is 10 Hz, then the ohmic value of X.sub.c is as follows: ##EQU1##
As a consequence, the input resistors must have values in excess of one megohm each. Inasmuch as the signal output level is quite low, effective transmission of this signal becomes very difficult, for noise pick-up and stray leakage due to moisture are then hard to avoid. With still lower operating frequencies or values of crystal capacitance, the impedance rises correspondingly to further aggravate this problem.
The Richardson, et al patent recognizes this problem and seeks to reduce stray capacitive coupling by placing an electrostatic shield around the piezo-electric element. However, in many applications, it is desireable that the system be free of temperature-sensitive and moisture-sensitive components. Also, with a sensor system of the type disclosed in this prior patent, the sensor is not effective at extremely low meter operating frequencies such as those occurring in very large vortex meters.